Thermocells in general are nonisothermal galvanic cells in which the electrodes are maintained at different temperatures and are in direct contact with an electrolyte which may be liquid or nonliquid solution. Temperature gradients and concentration gradients are present in conventional thermocells. The electrolyte contains ionic species for which the electrode reaction is reversible.
Such cells operate on a principle of mass flux against a temperature gradient that gives rise to a concentration gradient across each of the half-cells that comprise the basic cell unit. The concentration gradient generates as electromotive force and flow of electricity. Ions formed at the electrodes are transported between the half-cells to form an internal circuit. Oxidation occurs at the anode and electrons are given up and flow to the cathode through an external electrical circuit where positive ions recombine to form neutral materials.